The present invention relates to a fixing device equipped in, for example, an electrophoto-copying machine.
With regard to the fixing device of an electrophoto-copying machine or the like, it is required to maintain a fixing rate while a continuous copying operation. In order to maintain the fixing rate, it is necessary to control the temperature of the fixing device within a certain range during the continuous copying operation.
The fixing device feeds a sheet between a heat roller and a press roller pressed upon the heat roller to pass between them, and thus a toner image is fixed on the sheet. A heater is provided within the heat roller, and a thermistor is arranged on the surface of the heat roller.
The temperature of the fixing device can be controlled by, first, detecting the surface temperature of the heat roller using the thermistor, and turning on or off the heater within the heat roller in accordance with the temperature detected.
It should be noted that as the copy speed of the electrophoto-copying machine increases, each sheet passes through the fixing device in a short period of time, and therefore the heat amount given from the fixing device decreases.
Further, during the continuous copying mode, the number of sheets which pass through the fixing device within a certain period of time increases, and therefore the heat amount drawn by the sheets increases, thus lowering the temperature of the fixing device as a whole.
In order to avoid the lowering of the temperature, it is effective to maintain the temperature of the press roller.
The copying machine is set in a stand-by status when the surface of the heat roller of the fixing device becomes to have a predetermined temperature. At the same time, the surface of the press roller which is in contact with the heat roller is heated to the surface temperature of the heat roller.
However, when copying is started, sheets absorb heat of the heat roller and press roller, and further, the heat of the heat roller is absorbed not only by the sheets but also by the press roller which is in contact with the heat roller.
Under these circumstances, when the heat of the press roller which is once heated when a copying operation is started, can be kept, it becomes possible not only to decrease the heat amount absorbed from the heat roller to the press roller, but also to increase the heat amount given from the press roller to sheets.
An example of the technique of keeping the temperature of the press roller, is to enclose a press roller by a heat insulating member.
This technique is further divided into two versions. In one version, heat is reserved by a U-shaped bracket (sheet metal) which is provided to enclose a press roller, with a felt material is adhered on the inner surface of the bracket, and an aluminum foil adhered on the surface of the felt material, whereas in the other version, a press roller is enclosed by a U-shaped aluminum plate.
The U-shaped bracket or the U-shaped aluminum plate, mentioned above, is fixed to a fixation frame with a screw or the like.
However, when the U-shaped bracket or the U-shaped aluminum plate is fixed to the fixation frame with a screw, the radiation heat of the press roller is easily radiated to the fixation frame from the bracket or the aluminum plate via the fixation screw.
As a result, such a technique entails the drawback of a poor heat reserving effect of the press roller, which causes a decrease in the fixation rate in a high-speed copying mode or a continuous copying mode.